If three elements M, N and O are crystallized in cubic solid lattice with M atoms at corners, N atoms at cube centre & O atoms at the edges, then the formula of the compound is

Atom M is shared by 8 corners; 1 atom of O is shared by 4 unit cells. Atom N is present at centre of the unit cell. Hence, the effective number of atoms of M per unit cell = 8 X 1/8 = 1. The effective number of atoms of O per unit cell = 12/4 = 3.The effective number of atoms of N per unit cell = 1. Hence, the formula of the compound is MNO_3.